A glycoprotein that is very similar to interphotoreceptor retinoid-binding protein (IRBP) in molecular weight has been identified, and its binding of retinol and fatty acid and its recognition by goat anti-bovine IRBP have been revealed in 110,000 g supernatant prepared from the heads of wild-type Drosophila melanogaster, but not in supernatant preparations from eyes-absent (eya) mutants. This finding will provide the opportunity to study this protein in mutants which exhibit retinal degeneration. Binding studies using purified bovine IRBP indicate that palmitic acid alters the affinity of IRBP for retinol and that more than two retinol binding sites exist, one of which may be blocked in the presence of fatty acid.